This application incorporates by reference of Taiwan application Serial No. 90119362, filed on Aug. 8, 2001.
1. Field of the Invention
The invention relates in general to the voltage signal outputting apparatus, and more particularly to the apparatus for switching output voltage signals.
2. Description of the Related Art
Benefited from the advantages of the thinness, lightness and low radiation properties, LCDs (Liquid Crystal Display) have been widely used in the world.
FIG. 1 shows the block diagram of a conventional driver of a liquid crystal display. The color liquid crystal display panel 100 includes 1280×1024 displaying units. Each displaying unit includes three pixels for displaying red, green and blue respectively. Each pixel is controlled by a corresponding scan line and a corresponding data line. Thus, the whole panel includes 1024 scan lines and 1280×3=3840 data lines. When the panel displays a frame, a scan driver 104 can output a scan signal to enable each scan line in turn according to a first control signal (CTRL1). At the same time, a data driver 106 outputs analog pixel data signals (DATA) to the corresponding pixels respectively according to corresponding digital pixel data signals (DT), a second control signal (CTRL2) and gamma voltage input signals (GMV). The data driver 106 includes a nonlinear digital-to-analog converter (D/A converter) 108 for converting each digital pixel data signal (DT) to the corresponding analog pixel data signal (DATA) according to the gamma voltage input signals (GMV). The analog pixel data signal (DATA) is outputted to the corresponding pixel through the corresponding data line. The magnitude of the analog pixel data signal (DATA) determines the luminance (represented by gray level scales) of the pixel.
FIG. 2 shows the relationship between the gamma voltage of the liquid crystal display and the light transmittance of the pixel. The X-axis is indicative of the voltage of the lower plate and the Y-axis represents the light transmittance of the pixel. When upper plate voltage is Vcom, the lower plate voltage is called the gamma voltage. The electric potential difference between the gamma voltage and Vcom determines the light transmittance of the pixel. The relationship between the gamma voltage and the light transmittance of the pixel is not linear, but is like the gamma curve showed in FIG. 2 instead. Therefore, the lower plate voltage is called the gamma voltage. The magnitude of the gamma voltage influences the light transmittance of the pixel, but the polarity of the gamma voltage does not influence the light transmittance of the pixel. For example, when the gamma voltage changes from Va to Vb, the light transmittance of the pixel will not change, which is shown in FIG. 2. The nonlinear digital-to-analog converter 108 of the data driver 106 converts the digital pixel data signals (DT) to corresponding analog pixel data signals (DATA) according to the gamma relation between the gamma voltage and the light transmittance pf the pixel. The above-mentioned procedure is called gamma correction. The X-axis is indicative of the voltage of the lower plate and the Y-axis represents the light transmittance of the pixel.
FIG. 3 shows the gamma curve, which is for use in the data driver to perform gamma correction. The X-axis shows the magnitude the digital pixel data signals (DT) which are represented by binary numbers of six bits and the Y-axis shows the corresponding gamma voltages signals to the digital pixel data signals (DT). The gamma curve shown in FIG. 3 includes a positive polarity gamma curve 302 and a negative polarity gamma curve 304. Each digital pixel data signal (DT) corresponds to a positive polarity gamma voltage signal and a negative polarity gamma voltage signal. The points A, B, C, D and E chosen from the positive polarity gamma curve 302 and the points A′, B′, C′, D′ and E′ chosen from the negative polarity gamma curve 304 are reference points when performing gamma correction. According to the gamma curve shown in FIG. 3, each of the reference points corresponds to a gamma voltage input signal (GMV) and a digital pixel data signal (DT). The corresponding gamma voltage input signals (GMV) are reference gamma voltages while the corresponding digital pixel data signals (DT) are reference pixel data signals. When performing gamma correction, the nonlinear digital-to-analog converter 108 is to use the inner interpolation method to convert the digital pixel data signal (DT) to the gamma output voltage signal (DATA) according to those reference gamma voltages (GMV) and the reference pixel data signals (DT).
FIG. 4 shows a conventional apparatus for outputting the gamma voltage output signals. The conventional apparatus for outputting gamma voltage output signals is a string of resistors which is composed of a number of resistors (R0˜R31). The resistor string shown in FIG. 4 includes four input nodes for receiving the gamma voltage input signals (GMV) and thirty-four output nodes for outputting the gamma voltage output signals (DATA) respectively. When receiving the reference gamma voltages (V0˜V9) from the corresponding input nodes of the resistor string, each output node of the resistor string can output the corresponding gamma voltage output signal (DATA).
FIG. 5 shows the diagram of the driving circuit of the pixel P(N, M). The driving circuit of the pixel P(N, M) includes a thin film transistor T(N, M) and a pixel capacitor C(N, M). The gate electrode of the transistor T(N, M) is coupled to the scan line (SN) SN, the source electrode of the transistor T(N, M) is coupled to the data line (DN) DM, and the drain electrode of the transistor T(N, M) is coupled to the pixel capacitor C(N, M). When the scan driver enables the scan line (SN) SN, the transistor T(N, M) can be turned ON. At the same time, the analog pixel data signal (DATA) is delivered to the pixel capacitor C(N, M) through the data line (DN) DM and the transistor T(N, M). The luminance of the pixel P(N, M) is controlled by the voltage of the pixel capacitor C(N, M).
The gamma voltage output signal (DATA) inputted to the capacitor of each pixel has to be refreshed after every short period of waiting time. The period of the waiting time is defined to be the refresh rate of the display panel. If the refresh rate of the display panel is too slow, the magnitude of the pixel capacitor voltage will change due to the leaky current of the pixel. Therefore, the displaying color of the panel will change and the displaying frame will flicker. If the refresh rate of the display panel is too fast, the magnitude of power consumption of the driving circuit of the liquid crystal display will be enormous. To sum up, the disadvantage of the conventional gamma correction apparatus is unable to maintain the displaying performance of the panel and to reduce the total power consumption of the driving circuit at the same time.